Wind turbines may be arranged together forming a wind park, with a single point of connection, i.e. the PCC (“Point of Common Coupling”). Wind parks may comprise a substation including e.g. wind park transformers that convert power from the wind park voltage to a grid voltage. Such a substation may further include wind park control systems e.g. a supervisory control and data acquisition system (SCADA).
Wind parks may be arranged on land (“onshore”), or in the water (“offshore”), either as a plurality of floating wind turbines or wind turbines on pillars fixed in the sea bed.
It is known to provide an auxiliary power source, such as e.g. a diesel generator for supplying power to wind turbines when a connection with the grid is lost. Connection with the grid may be lost during planned maintenance, or during installation, but also during normal operation in case of a problem in the grid.
In a wind turbine, electrical components may be categorized into three levels, as follows: Level 1 electrical components are those components which are considered critical. These level 1 components are required to operate continuously, even during stand-by situations, i.e. when a wind turbine is disconnected from the electrical grid, and no electrical power is thus available.
Level 1 components may include components related to communication, and control and manoeuver of the gas insulated switchgear (GIS). It may further include air conditioning systems, sensors, PLCs, beacons and protective systems among others. The power required for these components may e.g. be around 15 kVA.
Level 2 electrical components may include those components which are less critical; these components may be required to operate only occasionally during stand-by situations. Level 2 components may include lubrication pumps, lighting systems, the service lift, pitch and yaw systems among others. The power required for these components may amount to e.g. around 30 kVA.
Finally, level 3 electrical components may not be required to operate during stand-by situations. Level 3 components may include e.g. cooling fans and pumps among others.
So at least for the level 1 components and at least occasionally for the level 2 components, electrical power supply must be available even in case of grid loss.
A plurality of auxiliary power generators, e.g. diesel generators, may be installed at individual wind turbines to be able to supply power to each wind turbine individually. If an auxiliary power generator is provided for each individual wind turbine, a problem is the high cost involved.
Alternatively, at least one more powerful auxiliary power generator may be provided at the central substation supplying all wind turbines simultaneously. If one auxiliary diesel generator is provided for a wind park, the logistics involved in providing fuel to each of the generators can be troublesome, in particular in the case of an offshore wind park.
Having a single more powerful auxiliary diesel generator providing electrical power to all wind turbines in the case of grid loss however may also have other disadvantages. One technical problem may be that the auxiliary diesel generator is over-dimensioned to take into account the in-rush of current at start-up, and the power losses associated with the reactive loads on the one or more transformers within each wind turbine.
One known approach is represented in FIG. 1. Reference sign 100 indicates the electrical grid, and reference sign 200 refers to an auxiliary power generator, for example a diesel generator. A switch 150 may be provided to alternatively connect the wind turbines either to the electrical grid or to the auxiliary power generator.
Each wind turbine may comprise a circuit breaker, which is commonly arranged in a gas insulated switchgear 900 for power ranges relevant to this sort of implementation, and a main wind turbine transformer 300. The main transformer may convert power from 66 kV as delivered from the grid or from the auxiliary power generator to 0.9 kV, the voltage level of the generator 600 of the wind turbine. The rating of the main transformer may be e.g. 6500 kVA.
A secondary wind turbine transformer 400 will further transform the power from 0.9 kV to the voltage level required by the electrical components of the wind turbine, such as e.g. lighting systems, pitch systems, pumps, cooling fans, etc. This voltage level may be 0.4 kV. The rating of the secondary wind turbine transformer may be e.g. 200 kVA in order to be able to feed electrical components of all levels (levels 1, 2 & 3).
The electrical components may be divided into level 1 components 800, which always need power supply and for which an interruption of the operation cannot be accepted and level 2 and 3 components 700, for which interruption of the operation is not necessarily problematic. To this effect, an uninterruptible power supply 500 (UPS) may be added to the level 1 critical electrical components' circuit. Additionally, circuit breakers may be arranged both for the level 1 circuit and for the levels 2 and 3 components. The circuit breaker 250 may be opened in case of grid loss, and only selectively closed so that only power is delivered to those components when needed.
In the event of an interruption of the power supply from the main grid 100, the uninterruptable power supply associated with the level 1 critical electrical components may continue to function for e.g. approximately 30 minutes. However beyond these 30 minutes, the auxiliary power generator will have to deliver the required power via the transformers.
At least one problem related with this kind of arrangement is the major power losses in the main transformer 300.
In examples of the present invention, at least some of the aforementioned problems are at least partially resolved.